The OPTIONS section of the configuration file allows the user to enable and disable various sub-models and calculations within the DIRSIG model. This might be done to simplify a run while an unrelated variable is studied or to improve run-time during scene construction process. Below is the OPTIONS section from the example CFG file.
#
# The OPTIONS section allows the user to enable and disable various
# aspects of the model. This includes some of the sub-models and/or
# advanced features.
#
OPTIONS {
ENABLE_TRUTH_IMAGES = FALSE
ENABLE_THERMAL_MODEL = TRUE
ENABLE_BRDF = TRUE
ENABLE_MAPS = TRUE
ENABLE_SOURCES = TRUE
ENABLE_PLUME_MODEL = TRUE
REMOVE_SENSOR_PATH = FALSE
USE_SCENE_PLATFORM_ANGLES = FALSE
GENERATE_IMAGE_PER_SCAN = FALSE
GENERATE_TRUTH_PER_SCAN = FALSE
SPECTRAL_LIBRARY_FILENAME = example.sli
}
Table 32-6 describes each of the variables that may appear in the OPTIONS section of the DIRSIG configuration file.
Table 32-6. List of the variables that can be set in the OPTIONS section.
| Variable Name | Default | Description | |
| USE_SCENE_PLATFORM_ANGLES | FALSE | Changes flight orientation angles to scene axis based | |
| ENABLE_TRUTH_IMAGES | FALSE | Controls creation of truth/debug image database | |
| ENABLE_THERMAL_MODEL | FALSE | Controls computation of surface temperatures | |
| ENABLE_BRDF | FALSE | Controls computation of complex reflected radiances | |
| ENABLE_MAPS | FALSE | Controls incorporation of material and texture maps | |
| ENABLE_SOURCES | FALSE | Controls computation of man-made source contributions | |
| ENABLE_PLUME | FALSE | Controls use of the internal JPL plume model | |
| REMOVE_SENSOR_PATH | FALSE | Removes the atmosphere between target and sensor | |
| SPECTRAL_LIBRARY_FILENAME | (none) | Filename of an ENVI Spectral Library file to create | |
| GENERATE_IMAGE_PER_SCAN | FALSE | Results in a separate image file for each instrument scan | |
| GENERATE_TRUTH_PER_SCAN | FALSE | Results in a separate truth image file for each instrument scan | |
| ENABLE_EARTH_ROTATION | FALSE | Controls skewing effects due to Earth rotation | |
| ENABLE_LINE_SKEW | FALSE | Controls skewing effects due to platform motion | |
| REGISTER_BANDS | TRUE | Forces registration of sensor bands | |
| REGISTER_DETECTORS | TRUE | Forces registration of offset detectors within a band | |
The general philosophy behind the default values set by DIRSIG for the various modeling options reflects a desire for the model to run quickly when the user configures a minimal simulation. As the user adds more and more complexity to a scene or simulation, the more rigorous (and computationally expensive) aspects of the model can be turned on to achieve the desired results.
The ENABLE_TRUTH_IMAGES variable controls the generation of the truth/debug image database discussed in Section 32.6. The default value requires the user to set this variable to TRUE to generate the truth image dataset defined by the default TRUTH_IMAGES file or the TRUTH_IMAGES section in their CFG file. The default value ensures that DIRSIG will not consume needless disk space by generating the truth image set that may not be utilized.
The ENABLE_THERMAL_MODEL variable controls the facet surface temperature computations incorporated into the thermal radiance solution. Running the thermal model increases DIRSIG's overall run-time and is therefore not desired for some cases. By default, this value is set to FALSE (to disable the thermal model).
The ENABLE_BRDF variable controls the level of complexity utilized in the reflected radiance computation. By default, DIRSIG will compute the incident radiance from the specular direction and assume that the background radiance in all other directions comes from the sky. If this variable is set to TRUE, then the actual incident radiance will be computed by sampling the hemisphere above the hit point. This sampling process involves a significant amount of additional ray-tracing and spectral calculations that will increase the run-time of the simulation.
Additionally, the complex surface reflectance computation determines the amount of sky exposure for each hit point. This sky exposure value is utilized by the thermal model to account for some radiational loading on the surface. Therefore, the default value implies that the all surfaces will have 100% sky exposure.
The modeling of material and texture maps is controlled by the ENABLE_MAPS variable. The default value disables any material or texture maps configured in the MAPS section of the CFG file (see Property Mapping Techniques). Therefore, if the user specifies any maps in the MAPS section, this variable needs to be set to TRUE.
Modeling material and texture maps will slightly increase the run-time of the model (by approximately 5%). Since the DIRSIG texture implementation utilizes a large set of reflectance/emissivity curves for each textured material, the memory requirements for a run will also increase when material and texture maps are used.
Setting the ENABLE_SOURCES variable to TRUE turns on the incorporation of any man-made or secondary sources configured in the SOURCES section of the CFG file . Since the computation of man-made source radiances requires additional ray-tracing to determine shadowing conditions and distances, enabling this option will increase the run-time of a simulation.
The path from the sensor to the ground contributes to the sensor-reaching radiance through transmission losses and scattered and emitted radiance. For some algorithm testing purposes, it may be desirable to remove these atmospheric components from the final imagery during the simulation. This would have the effect of simulating an atmospheric compensation that removes the contributions from this path (the downwelled sky terms are still included). The developer can then evaluate their algorithm without the errors and artifacts that may have resulted from a pre-processing stage, and isolate the performance of the algorithm. By setting the REMOVE_SENSOR_PATH variable to TRUE, the atmospheric contributions, along the path from the ground to the sensor, are removed.
If the user supplies the SPECTRAL_LIBRARY_FILENAME variable with a filename, DIRSIG will generate an ENVI format spectral library file containing the reflectances of all the materials used in the simulation. The spectral resolution and range will be defined by the sensor bandpass definitions configured by the user. If a material has more than one curve in the emissivity file, the average curve will be written to the spectral library.
If your organization uses the ENVI spectral tools (spectral unmixing, spectral angle mapper, matched filters, etc.) then this option can simplify the importing of reflectance spectra. The
By default, DIRSIG generates one image file for each spectral band (bandpass) defined in the input file. The spatial dimensions of each cube are driven by the size of the detector array (or the number of angular samples in a line-scanning system) and the number of scans in the entire flight. To simplify the processing of the spectral radiances predicted by DIRSIG with external software, the spatial radiance fields arriving at the sensor are saved to separate image files on a scan-by-scan basis. This option is activated by setting the GENERATE_IMAGE_PER_SCAN variable in the OPTIONS section to TRUE.
The compliment option for generating separate truth image files for each instrument scan is the GENERATE_TRUTH_PER_SCAN variable. The default value for this variable is FALSE which results in a single truth image file for all the instrument scans.
The ENABLE_PLUME_MODEL variable controls the use of the internal factory plume model configured in the PLUME section of the CFG file (see Gas Plume Modeling). If the user configures the needed variables to model a plume in their simulation, the ENABLE_PLUME_MODEL variable must be set to TRUE to realize the plume. Enabling the plume model requires some additional ray-tracing and temperature computations which will increase the run-time for the simulation.
By default, the platform orientation angles (roll, pitch and yaw) are measured with respect to the mean flight line (as recorded with inertial sensors). The orientation angles can be optionally interpreted as scene relative angles by setting the USE_SCENE_PLATFORM_ANGLES variable to TRUE. The details of all angular conventions regarding the platform for flight data is discussed in Classic Platform Data File.